The growth and differentiation of normal and malignant breast tissues is regulated by several growth factors. Prolactin (PRL), a known neuroendocrine hormone, is an important growth and differentiation factor for the human breast. We have recently shown widespread expression of PRL and its receptor (PRLr) in malignant and normal breast epithelium. These studies have also demonstrated that treatment of a breast cell line with anti-PRL antibody inhibited its proliferation and survival. these data indicate that breast epithelium utilizes PRL at the autocrine and endocrine level. The autocrine expression of PRL, therefore, may regulate the growth, differentiation, and pathology of the human breast. This hypothesis will be tested in this proposal by three specific aims using normal and malignant human tissues and cells. First, the function of autocrine PRL will be examined in breast tissues and cells in vitro under defined conditions. Second, the functional differences between dimers of PRLr isoforms involved in signal transduction will be investigated through the use of transfected homo- and hetero-dimeric receptor chimeras. Third, PRLr structure will be correlated with function through the in vitro and in vivo use of recombinant PRLr mutants. These aims will test whether the function and mechanism of action of autocrine PRL and PRLr contribute to the normal and pathologic development of the human breast. The studies proposed here will delineate this autocrine regulatory loop in human breast tissues and characterize the molecular basis for PRL-driven signal at interrupting the autocrine PRL loop in human breast cancer.